Synthesis of Li2Mn4O9 using lithium permanganate precursor

ABSTRACT

Li2Mn4O8+z, with z greater than zero and less than one, is prepared from LiMnO4 and an appropriate complimentary compound, such as MnOOH, MnO2 or MnCO3 precursors. The Li2Mn4O8+z is useful in highly oxidized lithium manganospinels.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein may be manufactured and used by or forthe government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the synthesis of Li₂Mn₄O_(8+z), with zgreater than zero and less than one, prepared from LiMnO₄ and anappropriate complimentary compound, such as MnOOH, MnO₂ or MnCO₃precursors, and its use in highly oxidized lithium manganospinels.

2. Brief Description of the Related Art

Lithium manganospinels have been investigated as lithium insertioncathodes for lithium-ion batteries (see e.g., A. de Kock, M. H. Rossouw,L. A. de Picciotto, M. M. Thackeray, W. I. F. David and I. M. Ibberson.Mater. Res. Bull. 25 (1990); M. M. Thackeray, A. de Kock, M. H. Rossouw,D. Liles, R. Bittihn and D. Hoge. J. Electrochem. Soc. 139 (1992); M. M.Thackeray and M. H. Rossouw. J. Solid State Chem.113 (1994); M. M.Thackeray, A. de Kock and W. I. David. Mat. Res. Bull. 28 (1993); M. M.Thackeray and M. H. Rossouw. J. Solid State Chem. 113 (1994); and R. J.Gummow, A. de Kock and M. M. Thackeray. Solid State Ionics 69 (1994)).The difficulty in preparing the fully oxidized material Li₂Mn₄O₉, in areproducible manner, is well known (see e.g., C. Masquelier, M. Tabuchi,K. Ado, R. Kanno, Y. Kobayashi, Y. Maki, O. Nakamura and J. B.Goodenough. J. Solid State Chem. 123 (1996)). Strict control ofexperimental conditions such as temperature, time, particle size of theprecursor materials, and oxygen partial pressure has been essential forproducing fully oxidized, single-phase material. Studies to date,however, show clearly that the fully oxidized Li₂Mn₄O₉ phase has neverbeen prepared successfully.

Many compounds and synthesis methods have been tried in attempting toproduce suitable cathode active materials for rechargeable lithiumbatteries. Compounds, which function as lithium insertion electrodes,include LiCoO₂ and LiTiS₂. However, lower cost and higher energy densitymaterials are desirable. In particular, lithium-manganese-oxides containmultiple attractive properties of high cell voltage, long shelf life,wide operating temperature and relatively non-toxicity (see e.g., M. M.Thackeray et al., J. Electrochem. Soc. Vol. 139, No. 2, February 1992).Ordinary manganese dioxide, MnO₂, cathodes, used in primarylithium/manganese dioxide 3 volt cell, show limited rechargeability inlithium cells. A significant improvement in capacity stability withcycling is obtained with cathodes prepared by reacting gamma MnO₂ withlithium hydroxide at moderate temperature (see e.g., N. Furukawa et al.in “Primary and Secondary Ambient Temperature Lithium Batteries”, J.Gavitno, Z. Takeharn and P Bro, Editors, PV 8-6, p. 557, TheElectrochemical Soc. Softbound Proc. Series, Pennington, N.J. (1988)).This reaction results primarily in the formation of a lithium-manganeseoxide spinel component with a cubic close-packed oxygen array, which hasbeen shown to be advantageous over one having a hexagonal close-packingarrangement (see e.g., M. M. Thackeray in “Proceeding of MRS Symposium,”boston, November/December 1988, Vol. 135, p. 585 (1989)).

SUMMARY OF THE INVENTION

The present invention includes a process for making Li₂Mn₄O_(8+z),wherein z is greater than zero and less than 1, comprising the steps ofmixing lithium permanganate with a precursor selected from the groupconsisting of MnOOH, MnO₂ and MnCO_(3,) and heating the mixtureeffective to form Li₂Mn₄O_(8+z), wherein z is greater than zero and lessthan 1.

The present invention also includes Li₂Mn₄O_(8+z), wherein z is greaterthan zero and less than 1, produced by the process of the presentinvention.

Furthermore, the present invention includes Li₂Mn₄O_(8+z), wherein z isgreater than 0.65, and Li₂Mn₄O_(8+z), wherein z is less than 0.88.

Additionally, the present invention includes a cathode comprising theLi₂Mn₄O_(8+z) made by the present process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a portion of the ternary phase diagram of the Li—Mn—Osystem;

FIG. 2 shows a TG curve of a LiMnO₄.3H₂O+MnCO₃ mixture at a heating rateof 2° C./min in an oxygen atmosphere;

FIG. 3 shows TG curves of (a) commercial LiMn₂O₄ cathode material; (b)highly oxidized Li₂Mn₄O_(8+z) material at a heating rate of 10° C./minin a helium atmosphere;

FIG. 4 shows a chart having chemical analysis of lithium manganospinelsof the present invention;

FIG. 5 shows a plot of the X-ray diffraction pattern of oxidized spinetof sample 5C in Table 1;

FIG. 6 shows a plot of the discharge behavior of Li₂Mn₄O_(8+z) cathodematerial: (a) 4th cycle; (b) 8th cycle of sample 4B at 0.1 mA/cm² in 1 MLiAsF₆/PC; (c) 1st cycle of sample 6C at 1.5 mA/cm² in 1 M LiPF₆ inEC/EMC;

FIG. 7 shows a comparison of capacity retention of spinel cathodematerials in coin cells of (a) Sample 5C and (b) LiMnO₄;

FIG. 8 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention;

FIG. 9 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention;

FIG. 10 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention;

FIG. 11 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention;

FIG. 12 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention; and,

FIG. 13 shows a TG analysis of the Li₂Mn₄O_(8+z) of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a highly oxidized lithium manganospinelusing Li₂Mn₄O_(8+z), with z greater than zero and less than one,prepared from solid state synthesis of LiMnO₄ and an appropriatecomplimentary compound, such as MnOOH, MnO₂ or MnCO₃ precursors. Thepresent invention also provides for novel process of making theLi₂Mn₄O_(8+z).

It is a desirable feature in choosing the lithium spinel structure forinsertion electrode is possession of a three-dimensional interstitialspace for Li⁺ transport, which remains intact over a wide compositionalrange as lithium is added. Selection of a stable spinel for theelectrode operating environment, while at the same time, choosing thedefect spinel with the highest insertion void volume maximizes thedeliverable energy density of a given cathode material.

As seen in FIG. 1, a portion of the ternary phase diagram of the Li—Mn—Osystem has those compounds containing Mn of near +4 valence in a shadedarea, e.g., Li₂Mn₄O₉ and higher oxides. These compounds possess thelarge void structure and high extraction potential of high energydensity rechargeable lithium cathodes.

The process of the present invention for making Li₂Mn₄O_(8+z) with zgreater than zero and less than 1, includes the steps of mixing lithiumpermanganate with a precursor of MnOOH, MnO₂ or MnCO₃ and heating themixture effectively to form Li₂Mn₄O_(8+z). Preferably, the lithiumpermanganate and precursor are mixed in approximately stoichiometricamounts. Stoichiometric amounts of lithium permanganate to MnOOH, MnO₂and MnCO₃ are 1:1, 1:1 and 1:1, respectively.

Preferably, the lithium permanganate comprises a hydrated form oflithium permanganate of LiMnO₄.3H₂O. Other forms of lithium permanganatemay be used as determined by one skilled in the art in light of thedisclosure herein. The lithium permanganate and precursor are mixed inthoroughly blended mixture, as more complete mixtures provideincreasingly efficient reactions to Li₂Mn₄O_(8+z). Mixing preferablyincludes grinding, with the grinding preferably at least as effective asfine grinding with a mortar and pestal combination. This mixing provideshomogeneity of the mixed constituents and intimate contact of theconstituents on a microscopic scale.

The lithium permanganate and precursor are heated, which results in amelt of the lithium permanganate and precursor. As the LiMnO₄.3H₂ Oprecursor forms the melt improved contact occurs among the reactants(see e.g., F. Leroux, D. Guyomard and Y. Piffard. Solid State Ionics 80(1995)). Lithium permanganate liquefies at about 160° C. as itdecomposes, allowing this transient phase to “wet” surrounding particlesof the precursor which additionally promotes intimate and homogeneouscontact among the constituents. Heating temperatures include from about275° C. or greater, preferably from about 275° C. to about 550° C., andmore preferably from about 350° C. to about 425° C. Higher temperaturessuch as from about 400° C. and greater, increasingly facilitate adecomposition of the Li₂Mn₄O_(8+z). Preferably, this heating step occurswithin an oxygen environment, such as an oxygen purge.

Li₂Mn₄O_(8+z), wherein z is greater than zero and less than 1, isproduced by the process of the present invention. Preferred values of zinclude z less than or equal to 0.88, z greater than or equal to 0.35, zgreater than 0.65, with more preferred values of z including z fromabout 0.65 to about 0.88. Significant variables for producing useful andconsistent quality material include: time, temperature, processingatmosphere starting constituent materials and mixinghomogencity/particle size.

The Li₂Mn₄O_(8+z), of the present invention is particularly usefulwithin a cathode. Preferably, the synthesis of spinel Li₂Mn₄O_(8+z)using a new precursor mixture LiMnO₄.3H₂O and MnOOH, MnO₂ or MnCO₃ isapplied in conventional manners, as determinable by one of ordinaryskill in the art.

The present invention provides a highly oxidized cathode spinelmaterial, i.e., to maximize the value of z in Li₂Mn₄O_(8+z), and tocharacterize the material. Thermogravimetry (TG) was employed todetermine the optimal experimental conditions for the synthesis of thecathode and the extent of oxidation of the product. The stoichiometrywas determined from potentiometric titration and elemental analysis ofLi and Mn. The superior electrochemical behavior of the material wasdemonstrated using coin cells and single cell tests in hermeticallysealed glass laboratory cells.

The lithium manganospinels spinels were prepared by solid-statereactions of stoichiometric amounts of thoroughly mixed powders ofLiMnO₄.3H₂O (manufactured by Fisher Scientific of Pittsburg.Pennsylvania, under the tradename Lithium Permanganate, Reagent Grade)and MnCO₃ (manufactured by Aldrich Chemical Co. of Milwaukee, Wis.,under the tradename Manganese Carbonate (99.9%)). The reactions wereinvestigated over a temperature range from 275 to 550° C. in a quartztube furnace under flowing oxygen. After the optimal temperature rangewas established, the effects of heating time, grinding, and oxidizingenvironment on the synthesis were investigated. The precursors wereheated at from about 2° C./min to about 5° C./min until the permanganateformed a melt, whereupon the temperature was held constant for about 15minutes. Heating was continued until the desired calcination temperaturewas reached. The effects of air, oxygen, ozone/oxygen and the additionof 30% hydrogen peroxide under flowing oxygen as oxidizing environmentswere examined.

LiMnO₄ may be synthesized by anion exchange resin procedure thatincludes passing a concentrated solution of Fisher Scientific CertifiedACS potassium permanganate through a column of Dowex ion exchange resin,HGRNGLi7 from Dow Chemical Company of Midland Mich. The resulting LiMnO₄solution was concentrated by evaporating the water. The product wasrecrystallized twice from distilled water, and dried under vacuum.

A TA Instruments, Inc. (of Newcastle, Del.), 951 thermogravimetricanalyzer was used in this study. The purity and moisture content of theprecursor materials were determined by TG measurements in a heliumatmosphere. TG measurements of precursor mixtures in oxygen were used todetermine the optimal temperatures and times for the bulk synthesis ofthe spinels in a tube furnace. The extent of the oxidation of the spinelwas determined by TG weight loss measurements in helium. Fouriertransform infrared (FTIR) spectroscopy was used to analyze the evolvedgases from the thermal decomposition of the spinel materials, using aNicolet 510P spectrometer interfaced to the TG instrument with a heatedtransfer line.

Chemical analysis of the composition was preformed. Powder X-raydiffraction (XRD) was used to evaluate the purity of the spinelmaterials. The composition was determined using direct current argonplasma emission spectroscopy (DCP) in combination with potentiometrictitration. Lithium content was determined by DCP. Manganese wasdetermined by potentiometric analysis. The oxygen content was determinedfrom the Li and Mn results. The oxidation state of Mn in the spinel wascalculated from analysis of the oxidizing power of Mn and the totalmanganese. The former was determined by dissolving the spinel in anexcess of acidified standard ferrous solution and titrating theremaining Fe²⁺ using standard KMnO₄. After addition of excesspyrophosphate to convert Mn²⁺ to Mn³⁺, total Mn was determined bypotentiometric titration with standard KMnO₄. The efficacy of thetitrimetric procedure was evaluated from analysis of standard LiMnO₄ anda standard ore of manganese. The presence of unreacted Mn²⁺ in thematerial could be detected by comparing the titrimetric results withthose found from the addition of known amounts of MnCO₃ to standardsamples of LiMn₂O₄.

Additionally, the composition was analyzed through electrochemicalevaluation. The electrochemical behavior of the spinel material wasdetermined in hermetically sealed glass lab cells containingapproximately 0.5 in.² electrodes. The lab cell consisted of a cathodesandwiched between two anodes and separated by two layers of Celgard.All components were assembled in a dry room. The electrolyte was either1 M LiAsF₆ in propylene carbonate or 1 M LiPF₆ in a 1:1 mixture ofethylene carbonate (EC) and ethylmethylcarbonate (EMC). The anode waslithium foil on nickel exmet. The cathode was prepared by dissolvingpolyvinylidene fluoride (PVDF) in a minimum amount of dimethylsulfoxide(DMSO), followed by adding a mixture of super-P carbon and theas-synthesized spinel. Additional DMSO was added to form a thick paste.

The composite cathode included spinel, carbon and PVDF in an 88:8:4 to85:10:5 weight ratio that was pressed onto Al exmet and dried overnightunder vacuum at 135° C. The cathode was initially charged, followed bycharge-discharge cycling under galvanostatic conditions. This cathodematerial was also cycled versus a lithium anode in a coin cell using 1 MLiPF₆ in electrolyte consisting of a 5:4:1 ratio of ethylenecarbonate-dimethylcarbonate-diethylcarbonate, i.e., EC-DMC-DEC.

A preliminary TG experiment was conducted to determine the sequence ofreactions in the synthesis of the Li₂Mn₄O₉ cathode material. A TG curveof a 1:1 stoichiometric mixture of LiMnO₄.3H₂O and MnCO₃ is shown inFIG. 2. The sample was heated to 450° C. at a rate of 2° C./min inoxygen. The first weight loss is attributed to the loss of 3 moles ofwater beginning about 60° C. The second weight loss at ˜150° C. wascaused by the decomposition of LiMnO₄ with the evolution of oxygen. Thisreaction is extremely exothermic, and slow heating rates and smallsample sizes are required to preclude the expulsion of the sample fromthe TG boat. The final product forms during the third weight loss withthe evolution of CO₂.

As seen in FIG. 2, a plot shows a TG curve of a LiMnO₄.3H₂O+MnCO₃mixture at a heating rate of 2° C./min in an oxygen atmosphere. Theproposed reactions for the three weight losses shown in FIG. 1 are givenby reactions (1), (2) and (3), below.

 LiMnO₄.3H₂O+MnCO₃→LiMnO₄+MnCO₃+3H₂O  (1)

LiMnO₄+MnCO₃→½Li₂O+MnO₂+MnCO₃+½O₂  (2)

Li₂O+2MnO₂+2MnCO₃+O₂→Li₂Mn₄O₉+2CO₂  (3)

Theoretical TG weight plateaus are calculated from the gravimetricfactors determined from the molecular weights of the solid phases. Theexperimental TG plateau values are determined from the ratio ofsuccessive plateaus. Thus, the experimental TG values for the threereactions are 0.8137, 0.8976, 0.8702. These measured values are inagreement with the theoretical values of 0.8167, 0.9003, and 0.8708. Themeasured weight plateau, 63.53%, for the overall reaction, shown inreaction (4) below, is in agreement with the theoretical value of64.03%, with the discrepancy attributed to the simultaneousdecomposition of the product at temperatures above about 400° C. At aheating rate of 2° C./min, the TG experiment lasted only about 220minutes. Therefore much longer reaction times, as described later, arerequired for the preparation of the highly oxidized spinel material.

2(LiMnO₄.3H₂O)+2MnCO₃→Li₂Mn₄O₉+2CO₂+6H₂O+½O₂  (4)

The solid-state synthesis of the fully oxidized Li₂Mn₄O₉ phase is highlydependent on the calcination time and temperature. If the temperature istoo low, it may not be possible to achieve a high degree of oxidation,even after several days of heating, because of the slower reactionkinetics. At higher temperatures, competition between the formation ofthe spinel and the reverse (decomposition) reaction can occur. Thiscritical temperature dependence was investigated by TG analysis. Anincompletely oxidized spinel was prepared by heating the precursors in atube furnace at 400° C. under oxygen. A portion of the material wastransferred to the TG apparatus and heated at a slow rate of 1° C./minin oxygen starting at ˜200° C. The TG behavior showed a steady increasein weight over the next 100 minutes as the spinel slowly oxidized. Nofurther oxidation was observed between about 340 and 400° C. Above 400°C., the spinel material began to slowly lose oxygen as it decomposedback to Li₂Mn₄O₈. This does not preclude decomposition of the spinel atlower temperatures provided sufficient calcination time is allowed. Adecrease in the oxidation state of Mn occurs in the spinel uponexcessive heating at 365° C.

As described in A. de Kock, M. H. Rossouw, L. A. de Picciotto, M. M.Thackeray, W. I. F. David and I. M. Ibberson. Mater. Res. Bull. 25(1990), Li₂Mn₄O₉ decomposes with a loss of oxygen when heated above 400°C. for long periods of time, according to reaction (5), below:

Li₂Mn₄O₉→2LiMn₂O₄+½O₂  (5)

It has been reported in J. Tarascon, W. Mckinnon, F. Coowar, T. Bowmer,G. Amatucci and D. Guyomard. J. Electrochem. Soc. 141 (1994), that athigher temperatures, the product of reaction (5), LiMn₂O₄, willdecompose according to reaction (6), below:

3LiMn₂O₄→3LiMnO₂+Mn₃O₄+O₂  (6)

The two-step thermal decomposition of the theoretical “Li₂Mn₄O₉”, shownby reactions (5) and (6) above, was characterized by TG analysis (seee.g., S. Dallek, in: Proceedings of the 38th Power Sources Conference,1998) and by X-ray studies. Separable TG weight plateaus wereestablished by selecting optimal experimental parameters such as smallsample size, slow heating rate and inert atmosphere. The oxygen contentof the starting material was then determined from the appropriategravimetric factors and the measured plateau values.

TG curves of a commercial LiMnO₄ sample and the highly oxidized Li₂Mn₄O₉material of the present invention are shown in FIG. 3. The samples wereheated at a rate of 10° C./min in an atmosphere of flowing helium. Theweight plateau ratio (94.04/99.94) in curve (a) for the LiMn₂O₄ sampleis in exact agreement with the theoretical value of 94.10% calculatedfrom reaction (6) above. The first weight loss for the highly oxidizedmaterial, curve (b), occurring from room temperature to about 300° C.,is due to the evolution of adsorbed and combined water. The secondweight loss, from about 300-600° C., is attributed to the decompositionof nominally named “Li₂Mn₄O₉” according to reaction (5) above. The thirdweight loss, from about 600-800° C., is attributed to the decompositionof LiMn₂O₄ according to reaction (6) above. Mn₃O₄ forms as a result ofthe decomposition of Mn₂O₃ at temperatures above 650° C. in an inertatmosphere. The values of non-horizontal weight plateaus were determinedfrom the minima in the TG derivative curves.

The oxygen content, z in Li₂Mn₄O_(8+z), was determined by equating themolecular weights of the solid phases to the TG weight plateaus asfollows:

Li₂Mn₄O_(8+z)→2LiMn₂O₄+Z/2O₂  (7)$\frac{2{LiMn}_{2}O_{4}}{{Li}_{2}{Mn}_{4}O_{8 + z}} = {\frac{361.62922}{361.6292} + {15.9994z}}$${{Therefore}\quad z} = {{\frac{95.17\%}{98.68\%}\quad z} = 0.83}$

The final weight loss, attributed to the known decomposition reaction ofLiMn₂O₄ (reaction (6)), was used as a corroborative measurement for theTG method. The theoretical plateau value, obtained from the ratio of themolecular weight of the products and reactants in reaction (6), is0.9410. As shown in FIG. 3, the experimental value, obtained from theratio of the final plateaus (89.49%/95.17%), was 0.9403. This agreementbetween the calculated and measured values provides strong evidence forthe validity of the TG method for determining the oxygen content in thenominal compound Li₂Mn₄O_(8+z). Implicit in the TG method is theassumption that the material is single phase and the atomic ratio of Mnto Li in the spinel phase is indeed 2. The method can be used for otherMn/Li ratios by adjusting the Li and Mn subscripts in reaction (7). Ifthe material is not single phase, then TG analysis will show significantdeviations from the theoretical value of 0.9410. The phase purity of thespinels was confirmed by XRD analysis. In highly oxidized spinels, suchas sample 6 (see Table 1, below), there was also agreement between theTG analysis and the chemical analysis.

The effect of synthesis temperature, time, and oxygen partial pressureon the extent of oxidation of the material was investigated bythermogravimetry (TG). X-ray diffraction (XRD), potentiometrictitration, thermogravimetry, and elemental analyses were used todetermine the stoichiometry of the product.

Derivative thermogravimetry (DTG) provides important insights into thethermal behavior of materials that are not readily apparent frominspection of the parent TG curve. For example, highly asymmetric orsplit peaks may be indicative of complex decomposition mechanisms or thepresence of extraneous phases. Such phases are usually undetectable byXRD because of the extreme similarity of the manganese spinel XRDpatterns. DTG curves of highly oxidized Li₂Mn₄O_(8+z) phases revealedsymmetric derivative peaks in the temperature range 300-600° C. Poorlyoxidized material, on the other hand, displayed asymmetric peaks or aprominent shoulder at 410° C. This can be caused by the simultaneousdecomposition of the unreacted MnCO₃ precursor. The evolution of CO₂ wasdetected by simultaneous TG/FTIR measurements using samples that hadlarge DTG peak shoulders. In general, CO₂ evolution correlated with thepeak at 410° C. This is virtually identical to the observed DTGdecomposition peak temperature of our MnCO₃ precursor material. A highlyoxidized “Li₂Mn₄O₉” material, having asymmetric DTG curve, showed noevidence of CO₂ evolution.

Spinel materials, as described later in Table 1 (see FIG. 4), wereprepared in oxygen by heating precursor mixtures for extended periods oftime, such as from about 17 hours to about 165 hours at 365° C. and 400°C. A typical X-ray diffraction (XRD) powder pattern for sample 5C,heated for 122 hours is shown in FIG. 5. The XRD data indicated thatonly a single phase was present. As cited by Xia and Yoshio (see i.e.,Y. Xia and M. Yoshio. J. Electrochem. Soc. 144 (1997)), however, smallamounts of impurities are difficult to distinguish by XRD. The latticeparameter for sample 5B, which is equivalent to Li_(1.97)Mn₄ ₀₀O₈ ₅₄,determined to be 8.142 Å. The oxidation state of manganese in thissample was determined by potentiometric titration to be +3.816. It iswell known (see e.g., C. Masquelier, M. Tabuchi, K. Ado, R. Kanno, Y.Kobayashi, Y. Maki, O. Nakamura and J. B. Goodenough. J. Solid StateChem. 123 (1996)) that the lattice parameter decreases with increasingmanganese oxidation state. Since attempts to prepare fully oxidizedLi₂Mn₄O₉ have never been demonstrated, it is believed that thesematerials have lattice parameters more in agreement with manganospinelshaving lower oxidation states.

Table 1, shown in FIG. 4, provides a chemical analysis of lithiummanganospinels of the present invention. The various sample of Table 1,Samples 1A-6C, were prepared by the solid state reaction of a one to onestoichiometric mole ratio of LiMnO₄ with MnCO₃. This mixture wasintimately ground and placed in a porcelain boat and heated slowly in atube furnace under a flowing atmosphere of oxygen gas at the citedtemperatures and calcination times. At several intervals the mixture wasre-ground and the calcination process continued.

Chemical analyses were performed on samples prepared under a variety ofconditions. Some representative analysis data are shown in Table 1. Thechemical analysis provides data for determining the total manganeseoxides present, and the average oxidation state of the manganese. Theefficacy of the analytical procedure was confirmed by determining thetotal percent manganese oxides in Li₂Mn₄O₉(Li₂O.4MnO₂), wherein thetheoretical value is 92.09% when all of the manganese is in the +4oxidation state.

Although stoichiometric precursor mixtures were used, most of the spinelproducts were found to have a Li/Mn ratio slightly greater than theintended 1:2 ratio. This is attributed to some residual water in theMnCO₃. Furthermore, the presence of unreacted MnCO₃ could adverselyaffect the analysis. When a known amount of MnCO₃ was added topre-analyzed commercial LiMnO₄, the Mn⁴⁺/Mn³⁺ ratio was dramaticallyreduced. This was not observed during analyses of spinels that weresynthesized under appropriate conditions, further confirming the TG-FTIRdata that the MnCO₃ had reacted completely.

Although the Li₂Mn₄O_(8+z) notation implies an “oxygen-rich” spinel, apositive value of z implies overall cation deficiency, such asLi_(1−x)Mn_(2−y)O₄. For these “oxygen-rich” spinels, the values of x andy in Table 1, represent the vacancies in the tetrahedral (8a) andoctahedral (16d) sites respectively, for a Li_(8a[Mn]) _(16d)O₄ spinel.For a unit cell LiMn₂O₄, the number of total tetrahedral sites (lithiumplus vacancies) is 1, and the total octahedral sites (Mn plus vacancies)is 2. The values of x and y may be calculated from valance balance. Whenthe ratio of Li/Mn in a compound containing 1 mole Mn is n, and theaverage oxidation state of Mn is m, a spinel of compositionLi_(2n)Mn₂O_(n+m) has 8a vacancy sites, x, given by 1-8n/(n+m) and 16dvacancies, y, given by 2-8/(n+m). A value of z, which shows the extentof oxidation of a spinel of nominal composition Li₂Mn₄O_(8+z), wascalculated from the ratio of Li/Mn in which the concentration of Mn wasnormalized to 4 moles in the spinel composition.

To determine the effects of different oxidizing environments, thesynthesis was performed at 400° C. for 17 hours in air, oxygen, and inan ozone-oxygen atmosphere. Some samples were further treated with 30%hydrogen peroxide. The data for samples 1A (air), 1B (O₂) and 1C (mix ofO₂ and ozone) in Table 1 show that oxygen was more effective than air inproducing an oxygen-rich spinel.

A lithium-rich spinel (sample 2) was produced when precursors wereheated under oxygen for 70 hours and then heated an additional 18 hourswith excess 30% hydrogen peroxide. Heating the sample for another 44hours with peroxide produced no additional oxidation. Based on theseresults, we selected oxygen as the optimal oxidizing atmosphere.

The effect of numerous intermittent grinding procedures during synthesisof the spinel was investigated. These syntheses were conducted at 390°C. since higher temperatures appeared to be detrimental to forming thehighly oxidized spinel. The analytical results for sample (3A), heatedfor 21 hours, are shown in Table 1. Analysis showed that the oxidationstate of Mn progressively increased through 36 hours of heating,achieving a value of +3.744. After the next sample was taken at 51hours, the oxidation state of Mn had decreased. Samples heated for <4hours displayed the multi-peak DTG behavior discussed earlier, with adominant MnCO₃ peak at ˜410° C. As the grinding and heating procedureprogressed to 8 hours and beyond, smoother DTG curves were observed. Theprocedure was repeated using a similar precursor mixture. The productswere analyzed after calcination for 20, 27, 39 (sample 4A), and 48(sample 4B) and 60 hours. The oxidation state of Mn was observed toreach a maximum after 48 hours. With these precursors, the best resultsrequired approximately 48 hours calcination at 390° C. The preparationof these materials in a reproducible manner is requires carefulselection of the precursors and the calcination temperature and time.

The effect of lower synthesis temperatures was investigated. Using small(˜3 g) samples, the decomposition of the product, represented by k_(d),began to exceed the rate of formation of the spinel, k_(f), after ˜48hours at 390° C. This is illustrated by the competing reactions of thesynthesis.

2LiMnO₄−2MnCO₃(κ_(ζ))→Li₂O+2MnCO₃+1.50₂→½O₂+Li₂Mn₄O₉+2CO₂(κ_(d))→2LiMn₂O₄+½O₂+2CO₂

To decrease the rate of the reverse (decomposition) reaction, thesynthesis procedure was carried out at 365° C. After intervals of 21,41, 67, 94, and 122 hours of intermittent grinding and heating at 365°C., the average oxidation state of Mn was 3.70, 3.70, 3.816, 3.782 and3.778 , respectively. Thus, it is clear that a maximum in the Mnoxidation state occurs despite the lower reaction temperature. Theanalytical data for 41, 67 and 122 hours (samples 5A-5C), are shown inTable 1. Table 1 shows that the average Mn oxidation state was higher inthe material synthesized at 365° C. than in the 390° C. materialalthough more time was required at 365° C. to achieve a highly oxidizedmaterial. Analysis showed the oxidation state of Mn increased withdecreasing reaction temperatures, from 400 to 390-365° C.

TG analysis showed that spinel materials picked up atmospheric moistureto different degrees. Whereas TG curves showed that LiMn₂O₄(Li₂Mn₄O₈)samples were completely anhydrous (FIG. 3a), spinels in which the valueof z>0 had a propensity to adsorb atmospheric moisture. As shown in FIG.3b, the highly oxidized spinel material contains a considerable amountof adsorbed and combined water that is evolved from room temperature toabout 250° C.

The effect of the permanganate water of hydration was examined. As shownin Table 1 spinels prepared from the anhydrous permanganate (6A-6C) weremore highly oxidized than those prepared from the hydrated precursor(5A-5C) when prepared under identical conditions. Compare 5A with 6A and5C with 6B. The composition of sample 6B was not determined by chemicalanalysis. Instead, the oxygen content of sample 6B was determined by TGto be z=0.85. From this result, and assuming a Li/Mn ratio of 1:2, theoxidation state of Mn, m, was calculated to be 3.91. These TG resultsare in very good agreement with the chemical analysis data for samples6A and 6C.

The electrochemical performance of several spinel materials samples 4B,5C, 6C, and commercial LiMn₂O₄, was evaluated as cathodes in prismaticlaboratory cells. The cycling performance of sample 4B was evaluatedusing 1 M LiAsF₆/PC electrolyte. During the initial cycling, thedischarge curves did not show any sharp voltage transition as might beexpected for a low crystalline defect spinel. The initial dischargerevealed a small 4 V plateau with a gradual transition into a 3 Vplateau. The initial capacity of the fourth cycle is shown in FIG. 6a.The discharge curve, shown in FIG. 6b, that occurs after eight shallowcycles at 0.1 mA/cm² displays primarily a 3.0 V plateau.

The cycling behavior was evaluated using 1 M LiP₆EC/EMC electrolyte inorder to increase the rate capability. Samples 5C and 6C were cycledbetween 4.5 and 2.5 V at 1.5 mA/cm². Very little initial chargeacceptance was observed in sample 6C because of the high oxidation stateof Mn in the spinel. This highly oxidized spinel now gives rise to twovoltage plateaus. The initial discharge capacity of sample 6C, shown inFIG. 6c, exceeded the 213 mAh/g expected for a 3-faraday reduction. Uponreplacing the lithium, separator and electrolyte after 10 cycles, an 8%increase in capacity was observed, indicating components other than thecathode contributed to some loss of capacity upon cycling.

In general, both the commercial LiMn₂O₄ and the spinel cathodes hereingave poor cycling performance in the prismatic cells. This loss ofcapacity was attributed to poor electrode compression. Thus, the cyclingbehavior of a cathode prepared from sample 5C was compared with asimilarly prepared LiMn₂O₄ cathode in coin cells, using a lithium anodeand EC-DMC-DEC electrolyte. The cells delivered 9 cycles before thelithium anode began to short each cell. In contrast to the LiMn₂O₄cathode, however, the cathode prepared from sample 5C retained capacityover the first few cycles. As shown in FIG. 5, the cathode made fromsample 5C provided excellent total capacity relative to the LiMn₂O₄cathode.

Highly oxidized Li₂Mn₄O₉ defect spinel materials have been prepared fromLiMnO₄ and MnCO₃ precursors. A z value of 0.88 in Li₂Mn₄O_(8+z) wasobtained. The materials were characterized using chemical, TG and XRDanalyses. Electrochemical discharge proved to have excellent capacity(>200 mAh/g) and capacity retention relative to LiMn₂O₄ when singlecells were cycled from 4.5 to 2.5 V in coin cells.

EXAMPLE 1

LiMnO₄.3H₂O and MnCO₃ were ground with a mortar and pestle and finelymixed together until homogeneous. Approximately 5 mg to 10 mg of themixture was used for TGA synthesis. A TGA synthesis processing of thefinely mixed LiMnO₄.3H₂O and MnCO₃ constituents included slow heating of5° C./min in O₂ to 160° C. followed by rapid heating Of about 30° C./minto 450° C., followed by rapid cooling to ambient temperature. The TGAsynthesis was done as a preliminary investigatory step. Unlike the otherexamples that used larger amounts of LiMnO₄.3H₂O and MnCO₃, i.e.,generally about 5 grams, the small amount of mixed LiMnO₄.3H₂O and MnCO₃resulted in a high surface to volume ratio that did not facilitatehomogeneous mixing during melt. The results are shown in FIG. 8. Thefirst weight loss resulted from the release of water of hydration, whichtheoretically is given by the ratio of 240.824/294.869=81.67%. Theactual residual of 81.34% indicated the possibility that the startingconstituents were not completely dry. Over the next two plateaus, apredicted mass change mass ratio of 0.9003, actually was 0.8976. Thispossibly indicated an insufficient oxygen pickup during the heating inO₂. The final reaction, anticipated at 64.03% was 63.53% as thetemperature of 450° C. is reached. This likely showed partialdecomposition of the final Li₂Mn₄O_(8+z) product at 450° C.

EXAMPLE 2

LiMnO₄.3H₂O and MnCO₃ were weighed in stochiometeric amounts andintimately ground in a mortar and pestle, placed in a porcelain boatinto a tube furnace under a stream of flowing oxygen. The LiMnO₄.3H₂Oand MnCO₃ mixture was heat to 160° C. at approximately 2° C./min to forma melt. The melt was heated in O₂ for approximately 60 hours at 400° C.The results are shown in FIG. 9. As seen in the TG curve of FIG. 9, thesample was heated at a rate of 10° C./min in a flowing atmosphere ofhelium. The first weight loss, from room temperature to about 300° C.,resulted from the release of water of hydration. The second weight loss,from about 300° C. to about 600° C., resulted from the decomposition ofthe Li₂Mn₄O_(8+z) product (Li₂Mn₄O₉→2LiMn₂O₄+½O₂). The third weightloss, from about 600° C. to about 800° C., is assumed to have resultedfrom the decomposition of LiMn₂O₄., according to3LiMnO₄→3LiMnO₂+Mn₃O₄+O₂. The plateau values were determined preciselyfrom a simultaneous plot of weight loss derivative curve. Knowing theLi/Mn ratio and oxidation state of Mn, the oxygen stoichiometry inLi₂Mn₄O_(x) was determined using TG weight plateaus and the appropriategravimetric factors. The final weight loss, attributed to the knowndecomposition reaction of LiMn₂O₄, used as a corroborative measurement.As seen in FIG. 9, measured weight plateaus for the decomposition ofLiMn₂O_(x) and LiMn₂O₄ are in close agreement with the theoreticalcalculated weight loss, give x a value of 8.46.

EXAMPLE 3

The procedures of Example 2 were followed with the exception that afterheating to 160° C., the melt was heated in O₂ for approximately 184hours at 400° C. in order to further decompose any residual carbonates.The results are shown in FIG. 10. The decomposition weight ratiosyielded an estimated x value of 8.59.

EXAMPLE 4

The procedures of Example 2 were followed with the exception that afterheating to 160° C., the melt was heated in O₂ for approximately 278hours at 400° C. The results are shown in FIG. 11. The decompositionweight ratios yielded an estimated x value of 8.58.

EXAMPLE 5

The procedures of Example 2 were followed with the exception that afterheating to 160° C., the melt was heated in O₂ for approximately 17 hoursat 400° C. in order to reduce any decomposition of the product. Theresults are shown in FIG. 12. The decomposition weight ratios yielded anestimated x value of 8.59.

Test electrodes, fabricated by pasting on perforated aluminumsubstrates, showed relative discharge capacities of from about 200ma-hrs/g or greater, such as up to approximately 230 ma-hrs/g (forapproximately 10 cycles), compared with approximately 135 ma-hrs/g forcommercial material having lithium metal anodes.

EXAMPLE 6

The procedures of Example 2 were followed with the exception that afterheating to 160° C., the melt was heated in O₂ with from about 0.5% toabout 1% ozone from an ozone generator for approximately 17 hours at400° C. in order to enhance oxygenization. The results are shown in FIG.13.

The foregoing summary, description, and examples of the presentinvention are not intended to be limiting, but are only exemplary of theinventive features which are defined in the claims.

What is claimed is:
 1. A process for making Li₂Mn₄O_(8+z), wherein z isgreater than zero and less than 1, comprising the steps of: mixinglithium permanganate with a precursor selected from the group consistingof MnOOH, MnO₂ and MnCO₃ in approximately stoichiometric amounts;heating said mixture to a temperature of from about 350° C. to about400° C. for of from about 40 hours to about 165 hours in an oxygenatmosphere; mixing intermittently said mixture of lithium permanganatewith said precursor; and forming Li₂Mn₄O_(8+z), wherein z is greaterthan zero and less than
 1. 2. The process of claim 1, wherein theprecursor comprises MnOOH.
 3. The process of claim 1, wherein theprecursor comprises MnO₂.
 4. The process of claim 1, wherein theprecursor comprises MnCO₃.
 5. The process of claim 1, wherein thelithium permanganate comprises LiMnO₄3H₂O.
 6. The process of claim 1,wherein the step of mixing results in a thoroughly blended mixture. 7.The process of claim 1, wherein the step of heating results in a melt.8. The lithium permanganate of claim 1 is anhydrous.
 9. A process formaking Li₂Mn₄O_(8+z), wherein z is greater than zero and less than 1,comprising the steps of: mixing anhydrous lithium permanganate withMnCO₃ in approximately stoichiometric amounts; heating said mixture to atemperature of from about 350° C. for of from about 165 hours in anoxygen atmosphere; mixing intermittently said mixture; and formingLi₂Mn₄O_(8+z), wherein z is greater than zero and less than 1.